Neoplastic cells as targets of spontaneously cytotoxic lymphocytes: studies with natural killer-like cell lines

Dendritic cells: making progress with tumour regression?

Due to their potent ability to activate the immune system, dendritic cells (DC) are showing promise as potential adjuvants for tumour immunotherapy of cancer patients. However, little is known about the effect tumour cells can have on DC function. Indeed, the discovery of different DC subsets with different immunological functions indicates that the relationship between tumour cells and tumour-infiltrating DC subtypes is likely to be complex. There remains a lot to be understood about the effects of tumours on DC before we can expect to benefit from DC-based tumour immunotherapy of cancer patients. Here we review the recent advances being made in understanding DC phenotype and function in relation to interactions with different types of tumours.

Production and characterization of a novel monoclonal antibody inhibitory for murine natural killer cell activity

Natural killer (NK) cells are considered to play an important role in tumor surveillance. The killing of tumor target cells by NK cells is the result of a complex series of sequential binding, signal processing and lytic events. However, the mechanism which NK cells use to recognize tumor targets is poorly understood. To further study the cell-surface molecules involved in tumor recognition, we immunized rats against cloned murine T cells with NK activity (DBA/2.1) and generated rat-mouse hybridomas which were screened for the ability to block lytic activity of DBA/2.1 effector cells. Culture supernatants from one IgM-producing hybridoma, designated S1C4, were found to consistently inhibit DBA/2.1-mediated lysis of YAC-1 target cells. Endogenous splenic NK activity was also diminished in the presence of S1C4 monoclonal antibody (mAb) while alloantigen-specific cytotoxic T lymphocyte (CTL) activity was not affected. S1C4 mAb appears to react with effector cell-surface structures involved in the recognition/adhesion phase of NK activity since pretreatment of effector cells with mAb S1C4 inhibits their ability to bind to YAC-1 target cells. ELISA studies revealed that the S1C4 antigen is expressed by a range of lymphoid cell lines, as well as by DBA/2.1 cells and fresh splenic NK cells. S1C4 mAb were shown to react with 22, 24, 30, and 46 kiloDalton (kDa) DBA/2.1 cell membrane components on immunoblots performed under reducing conditions. These structures do not correspond to any known recognition/adhesion molecules, suggesting that mAb S1C4 defines novel cell membrane components involved in NK cell function.

The role of T cell receptors in non-MHC-restricted cytotoxicity

The relationship between natural killer cells (NK) and cells of the T lineage has been obscured by the existence of poorly characterized clones of presumed NK origin. We have analyzed nine of these cloned cell lines displaying varying levels of cytotoxic activity against NKS YAC-1 target cells for rearrangement and expression of the genes encoding the alpha, beta, and gamma chains of the T cell receptor for antigen. Rearrangements at both the TcR beta and gamma loci were detected in all clones often at both alleles. Rearrangement patterns at the TcR beta locus were identical in several clones, despite different degrees of cytotoxicity. T cell receptor alpha, beta, and gamma genes were expressed as full length transcripts in all clones regardless of their levels of cytotoxic activity. To explore the involvement of cell surface molecules in the cytolytic events, studies were undertaken to determine whether cytotoxic activity could be inhibited by antibodies against CD3, LFA-I, and H-2KdDd. In two selected clones, both alpha and beta chains of the LFA-I molecule were expressed but only monoclonal antibodies against the alpha chain significantly blocked cytotoxicity. Cytotoxicity was also inhibited by monoclonal antibodies against epitopes of H-2KdDd and CD3, the extent of inhibition correlating with the level of surface expression on both clones. These data suggest that conventional alpha/beta heterodimers may be necessary but not sufficient for target cell recognition by these clones. Since T cell receptor rearrangement and expression occur normally in the T cell lineage but not the NK lineage, these results also indicate that a subpopulation of cells with non-MHC-restricted killer activity lies on the T cell differentiation pathway and is selected by in vitro growth with IL-2. The limited rearrangement pattern observed can be explained if only a small subpopulation of T cells is capable of non-MHC-restricted killing, and if certain rearrangements favor self-MHC recognition which is known to block cytolysis in the NK system.

Clonal analysis of antigen-specific interactions between T cells and genetically engineered B cells

In order to investigate T cell-B cell interactions we constructed monoclonal, antigen-specific T- and B-cell populations. The Ia+ B-cell lymphoma A20-2J was transfected with trinitrophenyl (TNP)-specific heavy (mu) and light (kappa) chain Ig genes. A hapten-carrier complex (TNP-keyhole limpet hemocyanin (KLH)) bound to the surface Ig expressed on the transfectant and was presented to carrier-specific T-cell hybridoma clones at markedly low doses of antigen (0.01 microgram/ml) and in an Ia-restricted fashion. Two responses were elicited in the responding T-cell clones: (i) high levels of IL-2 secretion (320 units/ml), and (ii) cytotoxicity directed against the antigen-presenting B cell. This cytotoxicity was inhibited by D-mannose and was directed against innocent bystander cells, unlike cytotoxicity mediated by NK cells or alloreactive cytotoxic T lymphocyte. Helper and cytotoxic functions were often present in different T-cell hybridomas but some clones exhibited both activities. One representative T-cell hybridoma exhibited strong helper function for TNP-primed splenic B cells as detected in a plaque-forming cell assay, but was cytotoxic toward antigen-presenting B cells. Such monoclonal assay systems for studying cognate interactions of heterogeneous T cells and specific antigen-presenting cells will provide us with valuable new approaches for the study of antigen-specific T-cell regulation of B-cell activation in immune responses.

Double restriction in NK cell recognition is linked to transmethylation and can be triggered by asparagine-linked oligosaccharides on tumor cells

The determinants of tumor cell susceptibility to NK cell-mediated cytolysis were analyzed in a two stage model. The binding of tumor cells to NK effectors was measured by target-effector conjugation and cold target competition in 51Cr-release assays, whereas triggering was measured by assaying phospholipid methylation in NK cells stimulated by intact targets. Representative targets could be grouped into three phenotypes based on the data. Those such as YAC 1.2 could bind and trigger NK cells whereas the mutagenized variant, YAC 6.28.8, could bind but was unable to trigger NK cells and therefore resisted lysis. The third phenotype was represented by HL-60 which could neither bind nor trigger NK cells and was therefore completely NK resistant. The oligosaccharide nature of the triggering molecules was demonstrated by showing that purified, high mannose containing, asparagine-linked oligosaccharides from tumor cell targets were potent stimulators of NK transmethylation at submicromolar levels. Tunicamycin pretreatment of target cells inhibited their triggering capacity but not their NK binding function. These results suggest a double restriction in NK specificity involving two independent but sequential stages in recognition represented in binding and triggering by asn-linked oligosaccharides on the tumor cell surface.

T cell receptor gene rearrangements in cells with natural killer activity in the mouse

Cell-mediated recognition can operate at different levels of complexity and specificity based largely on the time of appearance of effector mechanisms during the course of evolution. Antigen-specific cytotoxic T lymphocytes require both T cell receptor genes and lectin-like cell adhesion molecules (LFA-1, LFA-2, lymphocyte function-associated) to initiate and maintain stable effector target cell conjugates. Natural killer (NK) cells, on the other hand, do not require expression of T cell receptor genes in the recognition and killing of tumor cells and virally infected cells. Adhesion is mediated by a family of glycoprotein molecules, of which the LFA-1 and LFA-2 molecules appear as the most likely candidates. NK-mediated cytolysis proceeds in the absence of MHC restriction, but nevertheless appears to be triggered by depressed levels of self MHC products on the cell surface of target cells. Finally, interleukin 2-dependent, cloned cell lines with NK-like cytotoxic activity should no longer be considered as bona-fide NK cells but rather reclassified as a subset of T cells which displays NK function.

Sporadic somatic fusion between MDAY-D2 murine tumor cells and DBA/2 host cells: role in metastasis

An ouabain- and 6-thioguanine-resistant mutant (K3T103) of the metastatic MDAY-D2 murine tumor cell line was transplanted s.c. into syngeneic DBA/2 mice in order to isolate K3T103 X host cell hybrids emerging in vivo, and examine their metastatic potential. Of 10 tumor-bearing animals that were analyzed at various time intervals, only 3 developed fusion products in the primary site, present at a frequency ranging between 10(-5) and 10(-4). These hybrids survived in HAT + ouabain medium, had a lymphoblastoid morphology and a hypo-tetraploid karyotype, were non-adherent, and were as rapidly metastatic as K3T103 cells when transplanted s.c. or i.v. into DBA/2 mice. All these characteristics were shared by cloned hybrids generated in vitro following polyethylene glycol (PEG)-mediated fusion of K3T103 cells with normal DBA/2 normal splenocytes. In marked contrast, the products of K3T103 X DBA/2 normal lung fibroblast fusion displayed a fibroblastic appearance, were adherent, progressively ceased to divide and remained dormant for several weeks in culture. These results indicate: that spontaneous fusion of K3T103 cells with host cells in the course of their expansion and subsequent dissemination is a stochastic and rare event, and that since the expression of their tumorigenic and metastatic potential is retained after fusion with splenocytes, host cells of lymphoreticular origin are most likely involved in that process.